The upload course area is not for restoring a backup file. That is for creating courses with a csv file. If you want to restore a backup, go the front page of your site and select restore from the admin menu.
Oh that's interesting Richard - you just made me go and see what happens when you upload a file that isn't a CSV file. I just uploaded a random png image file instead, first thing I found, and I got the message "there is something wrong with the format of the CSV file - number of columns not constant". I think this is misleading if you have mistakenly uploaded a file like an .mbz. I wonder if we should change that wording to clarify? (I will go and change the "More information about this error" in the meantime...
